Various species of chlorine are used in small- and large-scale bleaching, oxidation, and disinfection, operations. These operations range from providing a weak sodium hypochlorite solution in a bottle for household whitening and disinfection (liquid bleach solutions, about 5% sodium hypochlorite), to delivering pure chlorine gas to a wastewater treatment plant waste stream. One problem with the use of pure chlorine gas, however, is its high toxicity and risk to workers case of leaks and accidents.
A common approach for large-scale water purification that can be safer than the transportation and subsequent on-site use of chlorine gas is the on-site production of chlorine dioxide. This strong oxidant is used for oxidation to disinfect water flows in drinking water treatment plants and in wastewater treatment plants. As a strong oxidant, chlorine dioxide destroys viruses, bacteria, and other microscopic organisms as it oxidizes compounds having a lower oxidation potential than itself. To maximize its oxidation and disinfection effects, in a water treatment system chlorine dioxide is preferably added after the sedimentation tank or basin.
Chlorine dioxide (ClO2; CASR n 10049-04-4) is a greenish-yellow gas at room temperature that is stable in the dark but unstable in the light. As noted, it is recognized as an extremely powerful biocide, disinfectant agent and oxidizer. As to regulatory allowance of chlorine dioxide in commercial and wastewater and water purification applications, in 1967, the United States Environmental Protection Agency (“EPA”) first registered the liquid form of chlorine dioxide for use as a disinfectant and sanitizer. In 1988, EPA registered chlorine dioxide gas as a sterilant.
Chlorine dioxide kills microorganisms by disrupting transport of nutrients across the cell wall. Chlorine dioxide is a gas, is highly soluble in water and smells like chlorine bleach. However, chlorine dioxide is not to be confused with chlorine gas. They are two distinct chemicals that react differently and produce by-products that also have little in common.
Chlorine dioxide, ClO2, offers the following benefits. First, ClO2 functions via an oxidative rather than chlorinating reaction, the mode of action of chlorine gas. This virtually eliminates the formation of chlorinated organic compounds that are suspected to increase certain cancer risks. Second, ClO2 when generated on site, eliminates the need for site storage of chlorine and/or transportation thereof.
Several types of chlorine dioxide generators are commercially available. Many still utilize gaseous chlorine in their generation process, and while effective, the risk management issues associated with chlorine still remain.